1. Field of the Invention
This disclosure relates to a time delay relay circuit for battery powered devices and, more particularly, to a relay having minimal battery drain when not in use.
2. Description of the Related Art
Time delay relays are used in many industries. The purpose of a time delay relay is to provide a mechanism for turning a device on or off after a predetermined amount of time. One example of a practical application for the time delay relay is for use in a motor vehicle, for example a tractor. Tractors are often left idle for a few months at a time and consequently small amounts of stand-by current flow through time delay relay circuits employed by the tractor. Over long periods of idle time the battery in the tractor is drained, preventing the operator from turning the engine when the tractor is once again needed.
A typical time delay relay 8 is shown in FIG. 1. A battery supply 10 has a positive terminal 12 and a battery ground 11. The positive terminal is connected to a momentary normally opened switch 16. The switch 16 is also connected to a enable input 34 of a timing circuit 30. Positive terminal 12 is connected to a first terminal 14 of an electromagnetic relay 18. Electromagnetic relay 18 has a second terminal 15 connected to a load 38. Load 38 is also connected to ground. Between the first terminal 14 and second terminal 15 is a movable contact which opens and closes according to a field generated by a relay coil 22. The first terminal 14 is also connected to a second input 28 of timing circuit 30. The timing circuit 30 has a first output 26 and a second output 32. Outputs 26 and 32 energize and deenergize the relay coil 22 to open and close movable contact 20. Timing circuit 30 also is connected to ground. Typically relays contain a suppression resistor 24 across the relay coil 22 to alleviate potential damage caused by switching transients.
Normal operation of the time delay relay circuit includes the closing of momentary normally open switch 16. This enables timing circuit 30 through enable input 34. The timing circuit draws power from positive terminal 12 through supply input 28. The relay coil 22 is energized through outputs 26 and 32. The movable contact 20 closes making a connection between first terminal 14 and second terminal 15. The timing sequence begins holding the contact 20 in a closed position for a predetermined amount of time. Closing contact 20 supplies power across the load 38 until the timing sequence expires.
This circuit has a drawback, however. When it is not in use it allows battery 10 to drain. Although current cannot pass through switch 16 when open nor can it pass across contact 20 when open. A path does exist for current leakage, however. When the circuit is not in use, i.e. stand-by mode, current from positive terminal 12 can flow to input 28 and than to ground 36. Although this current is small due to the high impedance path through timing circuit 30, over time battery 10 will drain. This can be a great disadvantage if, for example, a vehicle with a time delay relay in it remains standing idle for an extended period of time. The battery will drain, and when it is necessary to use the vehicle it will not start. Therefore, a need exists for a time delay relay that can reduce the amount of current flowing through the circuit to essentially zero amperes when the circuit is not in use.